A sputtering process deposits a thin film of target material on a substrate by dislodging small particles or atoms from a target, which coat the substrate. Magnetic fields have been used to enhance the sputtering process. For example, as disclosed in U.S. Pat. No. 4,166,018 to Chapin, titled “Sputtering Process and Apparatus,” which is incorporated herein by reference in its entirety, in a conventional vacuum sputtering process a substrate is placed in front of a sputtering cathode in a vacuum chamber. The sputtering cathode includes a substantially planar target formed of the target material. The pressure in the chamber is reduced and then, optionally, back filled with a reactive or inert sputtering gas. A negative voltage is applied to produce a plasma discharge from the target surface. The plasma discharge is intensified by magnets located behind the target to produce a closed loop magnetic field over the target surface.
Over time, the target becomes depleted. Unfortunately, uneven depletion of the target, e.g., grooving of the target surface, can adversely impact the quality of the film deposited on the substrate. It can also result in inefficient use of the target, with consequent cost penalties. One cause of uneven depletion of the target surface is that erosion is highest at regions where the magnetic field lines are tangent to the target surface. Thus, it is known in the art to rotate an array of magnets behind the target, to develop more uniform erosion of the target surface. U.S. Pat. No. 4,995,958 to Anderson et al., titled “Sputtering Apparatus with a Rotating Magnet Array having a Geometry for Specified Target Erosion Profile,” U.S. Pat. No. 5,248,402 to Ballentine et al., titled “Apple-Shaped Magnetron for Sputtering System,” U.S. Pat. No. 5,830,327 to Kolenkow, titled “Methods and Apparatus for Sputtering with Rotating Magnet Sputter Sources,” and U.S. Pat. No. 6,258,217 to Richards et al., titled “Rotating Magnet Array and Sputter Source,” each of which is incorporated herein by reference in its entirety, disclose various cathode assemblies for sputtering systems having a magnet array that rotates behind the plane of a circular flat target. It is also known to optimize the shape of the magnet array, either symmetric or asymmetric with respect to the axis of rotation, to further reduce uneven erosion of target surface.
Rotating magnet arrays create a moving deposition erosion zone at the stationary surface of the target and, correspondingly, a moving deposition plume from the surface of the target. This may result in an uneven deposition pattern on the substrate and a reduced ability to deposit precision films. This may be particularly problematic for certain sputtering processes, such as long-throw sputtering or long-throw reactive sputtering.
Further, the areas on the target that are not being eroded are subject to back coating and reactive gas poisoning. This can result in arcing and/or a reduced deposition rate. With moving magnets, it becomes problematic to cover areas of the target that are not getting eroded with shields that could prevent back-scattered coating material or reactive gases from building up a layer of undesirable material on the target surface. Typically, with moving magnets, the bigger the ratio of the target area to the magnet area, the worse these problems can become.
There is a need in the sputtering art to reduce non-uniform erosion of the target and provide improved target utilization. There is a further need to provide improved deposition rates and improved uniformity of the deposited films when depositing onto large-area substrates.